In focused ion beam (FIB) systems, ions are extracted from a source, formed into a beam, focused, and scanned across a substrate to form an image of a feature, to mill a feature or to deposit material from a gas ambient. As features become increasingly small, the FIB system must be optimized to provide a higher quality beam, that is, a smaller, more focused beam spot in which the distribution of current should be as compact as possible.
Several factors reduce the quality of the current distribution of the FIB. For ion columns using a liquid metal ion source (LMIS), a primary cause of reduced beam quality at low to moderate beam current is chromatic aberration. Gallium ions emitted from a liquid metal ion source have an energy distribution which is a combination of the intrinsic and particle interactions; the latter component is commonly referred to the as the Boersch affect. The former is very complicated as there are several different mechanisms to form the ions. Chromatic aberration is the result of particles of different energies being focused at different locations by the lenses in the ion column. The chromatic aberration causes the beam current distribution to vary with the energy spread (ΔE) of the ions. If the energies of the ions in an ion beam were plotted on a histogram showing the frequency of occurrence of ions at each energy value, the graph would have a peak at a “nominal” energy value, decrease rapidly for energies above and below the peak, and then taper off more slowly. The regions where the graph tapers off are known as the beam “tail.” The energy spread, ΔE, is typically measured as the “full width, half maximum,” that is, the energy between points at half the maximum peak value on either side of the peak. In a typical gallium liquid metal ion source, the energy spread in the beam having a current of 1 pA to several hundred nA is typically about 5 eV at an emission current of 1.5 to 2.5 μA from the source.
FIGS. 1A-1C are photomicrographs that show the effects of the beam energy tail on ion beam milling of photoresist. The features shown in FIGS. 1A-1C were milled using a gold-silicon ion source, with a beam current of 0.2 nA. In FIG. 1A, the beam was applied for two seconds to provide a dose of 4×104 ions per cm2. The beam was moved in a square pattern to mill a central square 100. The ions in the energy tail, having energies away from the peak were deflected differently in the ion column and fell outside the square, milling the photoresist lightly out to circle 102.
In FIG. 1B, the beam was applied for 10 seconds for a total ion dose of 2×1015 ions per cm2. The relative number of ions having a particular energy value decreases as the particular energy value is farther from the nominal beam value. That is, the number of ions gets smaller as the energy value gets farther from the nominal value. As the total number of ions is increased, however, ions having energies farther from the nominal value will also increase in number. The longer the milling operation, the more the effects of ions further in the energy tail will be seen. The circle 102 is wider in FIG. 1B than in FIG. 1A because ions further in the tail from the nominal value are having an increased effect because of their increased number. In FIG. 1C, the beam was applied for 100 seconds for a total ion dose of 2×1016 ions per cm2, and the circle 102 is even wider as the number of ions further away from the nominal energy value increases and the effects are more visible.
Monochromators are sometimes used to reduce the energy spread of electron beams. Monochromators are designed to truncate a symmetrical Gaussian-shaped energy distribution of the electrons in the beam by removing electrons having energies that diverge from the average energy by more than a specified amount. Monochromators are complex and typically greatly reduced beam current. Monochromators are therefore not employed in FIB systems used for charged particle beam milling and deposition in commercial applications.